Lost Unix v4 source code from 1973 recovered from decades-old magnetic tape

The oldest surviving Unix written in C is restored from a nine-track tape at the University of Utah

by · TechSpot

Serving tech enthusiasts for over 25 years.
TechSpot means tech analysis and advice you can trust.

Through the looking glass: A half-century-old magnetic tape containing the only known copy of Unix v4 has been found and recovered by the University of Utah's School of Computing. The nine-track 3M magnetic tape dates back to 1973 and contains roughly 40 megabytes of data – the earliest surviving Unix release in which both the kernel and core utilities were written in C.

Archivist Al Kossow of Bitsavers, who led the technical recovery, described the process as "easy" as such efforts go. The tape, he explained, had "a pretty good chance of being recoverable." It held up well for a medium that relies on a thin, coated film and precise magnetic signal integrity after decades of storage.

Kossow recovered the data by isolating the head read amplifier and using a multichannel, high-speed analog-to-digital converter. The system fed data into roughly 100 gigabytes of RAM before processing the signal with the readtape analysis program.

This setup captured raw analog signal data directly from the tape heads before converting and digitally reconstructing it, an approach designed to preserve even marginally readable signals.

The analysis tool, developed by computer historian Len Shustek, aligned the resulting waveform data into logical blocks representative of the original recording.

Once decoded, the archive produced the complete Unix v4 source tree and its associated artifacts. Roughly 40 MB of reconstructed data is now publicly available for download. The release includes a README file with step-by-step instructions for bootstrapping and compiling the system, intentionally reflecting how operating systems were prepared in the early 1970s.

// Related Stories

Unix v4 initially required a DEC PDP-11 minicomputer, a now-iconic model among early computing systems. Although the original hardware is long obsolete, enthusiasts can recreate the environment using SimH, an open-source emulator that emulates classic machine architectures, including the PDP-11.

Beyond the technical rescue, the team faced a puzzle of historical reconstruction. Unix was a young and experimental project in 1973, created by a small group of programmers exploring multipurpose time-sharing. The recovered media pointed to a recipient of the tape – Martin Newell, a computer graphics researcher at Utah best known for modeling the original Utah teapot. This 3D object later became a universal test model for rendering.

The recovered Unix v4 represents not only a historical milestone but also a rare touchpoint in software lineage. It captures the moment when operating systems transitioned from assembly to high-level languages, setting the stage for the portability and modular design that would come to define Unix and its descendants.